Thermal behavior and performance of shallow-deep-mixed borehole heat exchanger array for sustainable building cooling and heating
Introduction
Non-renewable energy sources are limited, but the development of modern society needs more energy sources or ways to use less energy for tasks. Concepts such as net zero energy buildings are being promoted increasingly and attracting much attention. In order to reduce carbon emissions and realize net zero energy buildings, it is important to conserve energy in buildings and make use of renewable energy [1]. Geothermal energy is widely available geographically and has large reserves, and there are many ways to use it [2], [3]. The ground source heat pump (GSHP) is a common technology of cooling and heating buildings using geothermal energy [4]. Ground heat exchangers transfer thermal energy to the ground in cooling season for storage, and extract thermal energy from the ground in heating season [5]. For shallow GSHP, systems are more mature and industrial, and have good and reliable technical specifications. Systems with deep borehole heat exchangers (DBHEs) can utilize deeper geothermal energy and may be more efficient than traditional shallow borehole heat exchangers (SBHEs). Research and experimentation on heat pumps with DBHEs are needed to enhance understanding of behavior and performance [6].
Research has been reported on deep borehole ground source heat pumps which usually extract/inject heat from/to the ground at depth around 1000 m to 3000 m, while conventional shallow BHEs only utilize geothermal energy at depth around 50 m to 200 m. Kohl et al. [7] assessed the advantages of DBHEs in geothermal energy utilization through monitoring operation data of a field project in Switzerland and establishing a numerical simulation optimization model. It was found that the efficiencies of DBHEs are higher than those of general shallow heat exchangers. Sapinska-Sliwa et al. [8] pointed out that, considering rising prices of energy resources and the accumulation of development experience on DBHEs, the DBHE may also be economically attractive in the future. Therefore, for the further development of DBHEs, it is necessary and worthwhile to model and study their performance and behavior. Cho et al. [9] indicate that, although the DBHE still has the limitation of high installation cost and that the research of the design and operation of DBHE are not enough, the use of relatively stable geothermal energy as a heat source provides considerable energy saving potential in heating systems.
For the study of DBHE's heat transfer model, analytical or numerical methods can be used to build the model [10], as shown below. The numerical method discretizes the solution area and solves it computationally [11], [12], [13]. Numerical methods are relatively accurate and reliable, but they have requirements for computer performance. Analytical methods start with the basic heat transfer process and derives the solution mathematically [14], [15], [16], [17]. Analytical methods can shorten the time needed to solve a problem to a certain extent. But it is quite complicated to derive an analytical model that is sufficiently accurate and realistic for use in design and other activities.
Most of the current research on ground energy systems focuses on the DBHE itself. The DBHE is similar to the SBHE, but no studies have been reported on GSHP with both a DBHE and a SBHE, and the possible interaction between them has not been well examined. In order to exploit the DBHE more fully and to allow its broader application, three significant points need to be considered, and these are now described.
- (1)
Difference in heating capacities for deep and shallow BHEs
Numerous analytical studies have been reported on shallow buried tube heat exchangers and deep buried tube heat exchangers. For SBHEs, Retkowski et al. [18] sought the optimal heat flux distribution by simulating four optimization methods (equal ground temperature distribution, maximal fluid temperature, maximal producible ground heat and maximal borehole wall temperature), and that the energy extraction in the best case increased by nearly 20%. Zhou et al. [19] when one combined the heat transfer capacity of a GSHP with an economic benefit model, so as to carry out a design optimization. Hein et al. [20] have assessed quantitatively shallow geothermal energy systems, and demonstrated that ground thermal conductivity, groundwater velocity, and borehole arrangement have great influences on performance.
For DBHEs, Huang et al. [21] analyzed effect of tube radius, tube depth and selected thermophysical parameters on the thermal performance of tube. Wang et al. [22] reduced total energy consumption of system by optimizing flow rate of tube on a daily basis. Huang et al. [23] rana long-term simulation of heat transfer performance of DBHEs for various operating modes. However, there is no detailed comparative analysis of heat transfer capacities of DBHEs and SBHEs.
- (2)
Design method of combined operation of deep and shallow BHEs
SBHE arrays can meet cooling and heating requirements at the same time. However, if cooling and heating loads are inconsistent, thermal energy in ground may be unbalanced, which can affect long-term operation efficiency of GSHP [24]. Other cold or heat sources can be added to bear the excess load and decrease the heat accumulation in the soil [25]. For instance, a zoning operation strategy according to load variation has been shown to be able to alleviate ground heat accumulation [26]. Also, controlling the water distribution around the GSHP tube can improve heat transfer rate [27].
Since ground temperature at 1000 m and greater is higher than that of the shallow soil, DBHEs are generally used to heat buildings. Deng et al. [28] proposed a design method to ensure long-term stable operation, mainly considering following key factors: geothermal conditions, DBHE parameters and heating season duration. Fu et al. [29] showed that by injecting heat into a medium borehole heat exchanger (MBHE) during non-heating period, heat transfer capacity could be increased. In other words, the DBHE may be operated as an auxiliary heat source. However, the feasibility of combining SBHEs with DBHEs is not discussed.
- (3)
Adaptivity and behavior of SD-BHE array for various end user demands
The scope of application of different GSHPs is not the same. For a given application, designers need to choose the right one according to the characteristics of different GSHPs. For SBHEs, Hu et al. [30] evaluated the suitability of GSHPs in the urban area of Linqu County, China through an analytic hierarchy process. Liu et al. [31] analyzed feasibility of GSHP in different climate zones through TRNSYS. Wang et al. [32] investigated development potential of surface-water heat pumps, groundwater-source heat pumps and ground-coupled heat pumps in Yangtze River Basin, China.
For DBHEs, Alimonti et al. [33] performed an energy research of a power plant containing a DBHE and suggested appropriate improvement measures. Holmberg et al. [34] analyzed overall performance of BHEs at depths of 300–1000 m. Bar et al. [35] considered feasibility of and design for storing excess heat from solar panels or thermal power stations using a DBHE.
However, the application of SD-BHEs has not been examined. Therefore, it is needful to explore for which ratio of cooling and heating load the SD-BHE array is suitable. Based on the above three points, this paper adopts a semi-analytical model to simulate and analyze SD-BHE systems. The innovation point of this paper is to compare difference of heat transfer capacity between a DBHE and a SBHE, and propose a design scheme combining the two for the first time. The purpose of this paper is to explore the feasibility of a SD-BHE array through the above simulation and analysis. In Section 2, the model is established. In Section 3.1, the difference of heat transfer capacity of a single tube under various cases, particularly shallow and deep locations, is discussed, which addresses point (1). In Section 3.3, points (2) and (3) are mainly discussed, including the proposed scheme of SD-BHE array. The analysis and comparison are carried out under three cold and heating load ratios. In addition, the influence of load distribution is described in Section 3.2, and the SD-BHE array is further analyzed in Section 3.4. In Section 4, the location of the deep tube and some limitations in the study are discussed. Finally, Section 5 analyses and summarizes the research results.
Section snippets
SD-BHE system description
The ground source heat pump system takes the ground as the heat source/cold source and the water as the medium. Through the heat pump, it dissipates heat for the building in the cooling season and provides heat for the building in the heating season. As shown in Fig. 1, heat exchanger boreholes in SD-BHE are arranged in a square array, and the deep tubes arranged at the four corners of the square array. A coaxial BHE is selected for the buried tube. In addition, SBHE and DBHE are connected in
Heating capacity difference for shallow and deep BHEs
For point (1) in section 1.2, in order to explore the gap between a DBHE and a SBHE in more detail, the depth, flow rate, surface temperature, geothermal gradient and ground thermal conductivity were taken as the research variables, and equivalent borehole number of tubes (EBN) was taken as the dependent variable. EBN represents how many times more heat can be extracted from a single DBHE than from a single SBHE under the same condition. A DBHE makes use of deeper geothermal energy, and its
Effect of the location of the deep tube
In the design of the SD-BHE array, the position of DBHE should be considered. In the above analysis, the DBHEs are placed at the four corners of the array by default. In this paper, the condition of Case 2 in section 3.3.1 is taken as an example to make a comparative analysis of changing the position of the DBHEs. The arrangement of DBHEs is shown in Fig. 13. Ten operating cycles are also simulated for the three SD-BHE arrays, as shown in Fig. 14.
In this example, the results for the three cases
Conclusions
GSHP can use renewable energy to cool and heat buildings. A DBHE has a strong heating capacity. The SBHE is widely used. However, the combined SD-BHE array has been inadequately examined. In this study, the performance of a SD-BHE array under various loads is simulated and analyzed. The main conclusions are as follows:
- (1)
The influence of some parameters is not entirely uniform between DBHE and SBHE. The heat extraction capacity per linear meter of DBHE is larger than that of SBHE. The depth of BHE
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study is supported by the National Natural Science Foundation of China (No. 52008182); National Key R&D Program of China (No. 2021YFE0113500); National Natural Science Foundation of China (No. 52278103); Key R&D Program of Hunan Province (No. 2022SK2086); Key R&D Program of Hubei Province (No. 2022BAA028); and the Open Project Program of Building Energy – Saving Engineering Technology Center in Anhui Province under Grant No. AHJZNX-2021-01.
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